1. Field of the Invention
The invention relates to a technique for equalizing state of charge variations among a plurality of electrical storage elements.
2. Description of Related Art
In Japanese Patent Application Publication No. 2001-218376 (JP 2001-218376 A), a voltage value of each of a plurality of single cells is detected and, when there are voltage value variations among the plurality of single cells, specified one or two or more of the single cells are discharged such that voltage value variations fall within an allowable range. Specifically, by discharging the single cells having a high voltage value, the voltage values of the plurality of single cells are caused to fall within the allowable range. Such a process is called equalizing process.
It is known that a full charge capacity of each single cell decreases with degradation of the single cell. When the full charge capacity has decreased, the state of charge (SOC) of the corresponding single cell also varies with a decrease in full charge capacity. The SOC is the percentage of a current amount of charge with respect to the full charge capacity. Therefore, when the full charge capacity varies, the SOC also varies.
In the technique described in JP 2001-218376 A, when there are SOC variations, specified one or two or more of the single cells are discharged such that the SOC in each of the plurality of single cells falls within the allowable range. In this case, even when the SOC varies due to full charge capacity variations, the equalizing process is carried out.
When the full charge capacity varies among the plurality of single cells, changes in SOC as a result of charging and discharging of each single cell differ depending on full charge capacity variations. In such a situation, when the equalizing process is carried out only on the condition that the SOC varies, electric energy stored in the single cells may be wastefully consumed.